A list punctuation question tests whether you can tell where one item ends and the next begins. Read around the blank to find the job: does the mark introduce a list or separate its items? Commas separate items with no commas inside them, while semicolons separate items that contain commas. A colon or a single dash can introduce a list after a full sentence. Then copy the separator the list already uses between its items.
Hints
- Hint 1
Read from “elevated levels of” to “and mercury.” Ask where the list begins.
- Hint 2
Check whether any item contains a comma of its own. Then use that fact to decide how the items should be separated.
Step-by-step
Find the list's separator
Step 1Locate the item boundary
Read from elevated levels of through and mercury to see what the blank needs to connect. Which names belong to the list? Every choice has phosphorus followed by nitrates, and mercury is the last substance. We're punctuating three separate list items, so the mark after phosphorus sits between the first and second rather than before the list.
- Step 2
Test what a colon or dash would do
A colon or single dash can introduce a list after a full sentence when the list spells out something in that sentence. Could nitrates and mercury spell out what phosphorus means? No, because all three substances belong after elevated levels of. So the next names continue the list, even though the sentence could end after phosphorus. The version with a colon and the version with a dash would turn those next items into an explanation of the first.
- Step 3
Copy the separator
Which separator does the completed list already show? Every choice places a comma after nitrates, before and mercury. None of the substance names has a comma inside it, so commas separate these items; semicolons separate items that contain their own commas. The semicolon version breaks that pattern, so use phosphorus, nitrates,. Choice D.